Phase shift mask including sub-resolution assist features for isolated spaces

ABSTRACT

A method extends the use of phase shift techniques to complex layouts, and includes identifying a pattern, and automatically mapping the phase shifting regions for implementation of such features. The pattern includes small features having a dimension smaller than a first particular feature size, and at least one relatively large feature, the at least one relatively large feature and another feature in the pattern having respective sides separated by a narrow space. Phase shift regions are laid out including a first set of phase shift regions to define said small features, and a second set of phase shift regions to assist definition of said side of said relatively large feature. An opaque feature is used to define the relatively large feature, and a phase shift region in the second set is a sub-resolution window inside the perimeter of the opaque feature.

RELATED APPLICATION DATA

[0001] The present application is a continuation-in-part of applicationSer No. 09/669,367, filed Sep. 26, 2000 entitled Phase Shift MaskSub-Resolution Assist Features; which application claims the benefit ofprior U.S. Provisional Application No. 60/215,938; filed Jul. 5, 2000;entitled Phase Shift Masking for Complex Layouts, invented by ChristophePierrat, which is incorporated by reference as if fully set forthherein.

[0002] The present application is related to co-pending U.S. patentapplication Ser. No. 09/669,368, entitled Phase Shift Masking forIntersecting Lines, invented by Christophe Pierrat, filed Sep. 26, 2000,and owned by the same assignee now and at the time of invention; andalso related to co-pending U.S. patent application Ser. No. 09/669,359,entitled Phase Shift Masking for Complex Patterns, invented byChristophe Pierrat, filed Sep. 26, 2000, and owned by the same assigneenow and at the time of invention.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to manufacturing small dimensionfeatures of objects, such as integrated circuits, usingphotolithographic masks. More particularly, the present inventionrelates to the application of phase shift masking to complex layouts forintegrated circuits and similar objects.

[0005] 2. Description of Related Art

[0006] Phase shift masking, as described in U.S. Pat. No. 5,858,580, hasbeen applied to create small dimension features in integrated circuits.Typically the features have been limited to selected elements of thedesign, which have a small, critical dimension. Although manufacturingof small dimension features in integrated circuits has resulted inimproved speed and performance, it is desirable to apply phase shiftmasking more extensively in the manufacturing of such devices. However,the extension of phase shift masking to more complex designs results ina large increase in the complexity of the mask layout problem. Forexample, when laying out phase shift areas on dense designs, phaseconflicts will occur. One type of phase conflict is a location in thelayout at which two phase shift regions having the same phase are laidout in proximity to a feature to be exposed by the masks, such as byoverlapping of the phase shift regions intended for implementation ofadjacent lines in the exposure pattern. If the phase shift regions havethe same phase, then they do not result in the optical interferencenecessary to create the desired effect. Thus, it is necessary to preventinadvertent layout of phase shift regions in phase conflict.

[0007] Another problem with laying out complex designs which rely onsmall dimension features, arises because of isolated exposed spaceswhich may have narrow dimension between unexposed regions or lines.

[0008] Because of these and other complexities, implementation of aphase shift masking technology for complex designs will requireimprovements in the approach to the design of phase shift masks, and newphase shift layout techniques.

SUMMARY OF THE INVENTION

[0009] The present invention provides techniques for extending the useof phase shift techniques to implementation of masks for complex layoutsin the layers of integrated circuits, beyond selected critical dimensionfeatures such as transistor gates to which such structures have beenlimited in the past. The invention provides a method that includesidentifying features for which phase shifting can be applied,automatically mapping the phase shifting regions for implementation ofsuch features, resolving phase conflicts which might occur according toa given design rule, and applying sub-resolution assist features withinphase shift regions. The present invention is particularly suited toopaque field phase shift masks which are designed for use in combinationwith binary masks defining interconnect structures and other types ofstructures that are not defined using phase shifting, necessary forcompletion of the layout of the layer.

[0010] Various aspects of the invention include computer implementedmethods for definition of mask layouts for corresponding complex layoutsin the layers of integrated circuits to be made using such masks,methods for manufacturing masks having such mask layouts, methods formanufacturing integrated circuits having improved small dimensionfeatures implemented using the novel masks, and improved integratedcircuits having the improved small dimension features.

[0011] The invention includes a method for producing photolithographicmasks, and layout files for such photolithographic masks, whichcomprises identifying features in a pattern to be exposed having adimension less than a particular feature size, and laying out phaseshift regions using a layout rule for the identified features to producea phase shift mask having phase shift areas. The particular feature sizeaccording to the invention need not be the critical dimension for thesmallest features to be implemented. Rather, in the layout of an entirecomplex pattern, any feature which is suitable for implementation usingphase shifting can be identified according to the present invention.

[0012] In one embodiment, the pattern to be laid out includes featureshaving a dimension smaller than a first particular feature size (e.g., anarrow line), and at least one relatively large feature, such as a flagshaped feature. In the pattern according to this embodiment, the atleast one relatively large feature and another feature (e.g. a line oranother flag shaped feature) in the pattern have respective sidesseparated by a space between said respective sides, said space having adimension less than a second particular feature size. Phase shiftregions are laid out for the identified pattern to produce a phase shiftmask, the phase shift mask having a first set of phase shift regions todefine said features having a dimension smaller than a particularfeature size, and a second set of phase shift regions to assistdefinition of said side of said relatively large feature.

[0013] In one embodiment, the process of identifying features suitablefor implementation using phase shifting includes reading a layout filewhich identifies features of the complex pattern to be exposed.

[0014] In one preferred embodiment, the phase shift mask includes anopaque field, and the phase shift regions include a plurality oftransparent regions having a first phase within the opaque field, and aplurality of complementary transparent regions having a second phase 180degrees out of phase with respect to the first phase, within the opaquefield. The opaque field leaves unexposed lines formed by the phase shiftregions unconnected to other structures. A complementary mask is laidout for use is conjunction with the opaque field phase shift mask toform interconnect structures in the region blocked by the opaque field,so the features formed using the phase shift mask are integrated withlarger dimension features. In one embodiment, the complementary mask isa binary mask, without phase shifting features.

[0015] As a result of the layout rule, regions in the phase shift maskmay result in phase conflicts. Thus, the invention also includesapplying an adjustment to one or more of the phase shift regions in thephase shift mask to correct for phase conflicts. The adjustment in onepreferred embodiment comprises dividing a phase shift region having afirst phase into a first phase shift region having the first phase in asecond phase shift region having the second phase. An opaque feature isadded to the phase shift mask between the first and second phase shiftregions. The complementary mask includes a corresponding opaque featurepreventing exposure of the features to be exposed using the first andsecond phase shift regions in the phase shift mask, and includes acut-out over the opaque feature separating the first and second phaseshift regions to expose any feature resulting from the phase differencebetween the first and second phase shift regions. In one embodiment, theunique structure which results from the adjustment is laid out in thefirst instance to prevent phase conflicts in the layout, and so may notbe considered an “adjustment” to correct a phase conflict in the layout.

[0016] For example, phase conflicts can arise in the implementation of apattern consisting of an intersection of an odd number of line segments.The odd number of line segments defines a plurality of corners at theintersection. In this case, phase shift regions are laid out adjacentthe line segments on either side of the corner so they have the samephase, and preferably continuing around the corner in all of theplurality of corners, except one. In one excepted corner, a first phaseshift region having the first phase is laid out adjacent the linesegment on one side of the corner, and a second phase shift regionhaving the second phase is laid out adjacent the line segment on theother side of the corner. An opaque feature is added between the firstand second phase shift regions in the one corner. The complementary maskincludes a corresponding opaque feature preventing exposure of theintersecting line segments left unexposed by the phase shift mask, andincludes a cut-out over the opaque feature separating the first andsecond phase shift regions to expose any feature resulting from thephase difference in the one excepted corner between the first and secondphase shift regions.

[0017] The selection of the one excepted corner having the cut-outfeature in the structure that defines the intersection of an odd numberof line segments is implemented in various embodiments according todesign rules. In one design rule, the one excepted corner is the cornerdefining the largest angle less than 180 degrees. In another designrule, the one excepted corner is the corner which is the greatestdistance away from an active region on the integrated circuit.

[0018] In one embodiment, the pattern to be implemented includes exposedregions and unexposed regions. Exposed regions between unexposed regions(i.e., spaces between lines or other structures) having less than aparticular feature size are identified for assist features. Theparticular feature size used for identification of exposed regionsbetween unexposed regions may or may not be the same as the feature sizeused for selection of unexposed regions (i.e., lines) to be implementedusing phase shift masking. According to this aspect of the invention,the process includes laying out phase shift regions in the phase shiftmask to assist definition of edges of the unexposed regions betweenexposed regions.

[0019] According to another aspect of the invention, the processincludes adding sub-resolution assist features inside a particular phaseshift region in the phase shift mask. The sub-resolution featurescomprise in various embodiments features inside and not contacting theperimeter of the particular phase shift region. In other embodiments,the sub-resolution features result in division of a phase shift regionhaving a first phase into first and second phase shift regions havingthe same phase. An opaque feature between the first and second phaseshift regions acts as a sub-resolution feature to improve the shape ofthe resulting exposed and unexposed regions.

[0020] The sub-resolution features do not “print” in the image beingexposed, but affect the intensity profile at the wafer level, such as byimproving contrast of the image and thereby improving process latitude,and changing the size of the printed image caused by the phase shiftregion in which the sub-resolution feature is laid out, such as foroptical proximity correction OPC.

[0021] According to another aspect of the invention, the layout of phaseshifting regions in an opaque field includes a step of simulating anintensity profile or other indication of the exposure pattern to begenerated, and locating regions in the exposure pattern which areanomalous, such as by having higher intensity. Sub-resolution featuresare then added to the layout covering the anomalous regions in theexposure pattern.

[0022] The use of sub-resolution features within phase shift regions isapplied uniquely for the formation of an array of closely spaced shapes,such as an array of capacitor plates used in dynamic random accessmemory designs.

[0023] An overall process for producing a layout file, or aphotolithographic mask is provided that includes identifying features tobe implemented using phase shifting, laying out phase shifting regionsso as to prevent or minimize phase conflicts, applying sub-resolutionassist features to the phase shift regions, and producing a layout file.Next, a complementary mask is laid out to complete definition of theexposure pattern so that features that are not implemented using thephase shift mask are interconnected with the features implemented by thephase shift mask.

[0024] A method for producing integrated circuits having improved smalldimension structures includes applying a photo-sensitive material to awafer, exposing the photosensitive material using the phase shift maskimplemented as described above, exposing the photo-sensitive materialusing the complementary mask implemented as described above, anddeveloping the photo-sensitive material. A next process step in themethod for producing integrated circuits involves the removal ofmaterial underlying the photo-sensitive material according to theresulting pattern, or addition of material over the wafer according tothe pattern resulting from the use of the phase shift and complementarymasks. The resulting integrated circuit has improved, and more uniformline widths, and improved and more uniform spaces between structures onthe device. In some embodiments, the resulting integrated circuit hasintersecting lines defined with phase shift masks.

[0025] The invention results, therefore, in methods for producing masklayout files and photolithographic masks based on such layout filessuitable for the implementation of complex designs extensively usingphase shifting structures to define small dimension features. Newmanufacturing techniques and improved integrated circuits are thereforeprovided.

[0026] Other aspects and advantages of the present invention can beunderstood with review of the figures, the detailed description and theclaims which follow.

BRIEF DESCRIPTION OF THE FIGURES

[0027] The patent or application file contains at least one drawingexecuted in color. Copies of this patent or patent applicationpublication with color drawing(s) will be provided by the Office uponrequest and payment of the necessary fee.

[0028]FIG. 1 illustrates a binary mask and FIG. 2 illustrates a phaseshift mask according to a prior art phase shift masking technique.

[0029]FIG. 3 is a plot of the intensity profile of an exposure madeusing the masks of FIGS. 1 and 2 according to the prior art.

[0030]FIG. 4 illustrates a binary mask, and FIG. 5 illustrates a phaseshift mask according to the present invention for implementing the sameshape as implemented with FIGS. 1 and 2.

[0031]FIG. 6 is a plot of the intensity profile of an exposure madeusing the masks of FIGS. 4 and 5 according to the present invention.

[0032]FIG. 7 is a binary mask, and FIG. 8 is a phase shift mask forimplementation of a feature comprising three intersecting line segmentsaccording to the present invention.

[0033]FIG. 9 is a plot of the intensity profile of an exposure madeusing the masks of FIGS. 7 and 8.

[0034]FIG. 10 is a binary mask, and FIG. 11 is a phase shift mask forimplementation of a feature comprising five intersecting line segmentsaccording to the present invention.

[0035]FIG. 12 illustrates a phase shift mask for implementation of adouble “T” structure.

[0036]FIG. 13 illustrates an alternative phase shift mask forimplementation of a double “T” structure according to the presentinvention.

[0037]FIG. 14 illustrates one example of the layout of a phase shiftmask according to the present invention for a complex pattern.

[0038]FIGS. 15A and 15B illustrate the layout, a simulation, and contourplots of a prior art phase shift mask for implementation of a densearray of capacitor plates on integrated circuit.

[0039]FIGS. 16A and 16B illustrate the layout, a simulation, and contourplots of the phase shift mask for implementation of a dense array ofcapacitor plates on an integrated circuit according to the presentinvention.

[0040]FIG. 17A illustrates a phase shift mask having sub-resolutionassist features, for implementation of a exposure pattern as shown inFIG. 17B.

[0041]FIG. 17B shows an exposure pattern which results from the phaseshift mask of FIG. 17A, and an exposure pattern which would result fromthe phase shift mask of FIG. 17A without the assist features.

[0042]FIG. 18 is a flow chart of a process for producing layout files,and phase shift mask and manufacturing integrated circuits according tothe present invention.

DETAILED DESCRIPTION

[0043] A detailed description of the present invention is provided withrespect FIGS. 1-18. FIGS. 1-3 illustrate problems associated with thelayout and manufacturing of small dimension features according to theprior art. FIGS. 4-6 illustrate an approach to improving the layout andmanufacturing of the small dimension features shown in FIGS. 1-3according to the present invention. FIGS. 7-18 illustrate additionalfeatures and techniques.

[0044]FIG. 1 shows a binary mask for use in combination with an opaquefield phase shift mask as shown in FIG. 2. The binary mask of FIG. 1includes an opaque feature within a clear field 10. The opaque featureincludes a blocking region 11 which corresponds to the features, i.e.transistor gates in an active region of a device, formed using the phaseshift structures of FIG. 2. Narrow lines 12, 13 and 14 extend from theblocking region 11 to respective flag shaped elements 15, 16, 17. Thenarrow lines 12, 13, 14 in this example each extend through the blockingregion 11, resulting in respective extension portions 18, 19, 20. Thephase shift mask of FIG. 2 is formed within an opaque field 25, insidewhich zero degree phase shift regions 26, 27 and 180 degree phase shiftregions 28, 29 are formed. The phase shift regions result in theprinting of fine lines on the transitions between zero degree region 26and 180 degree region 28, between 180 degree region 28 and zero degreeregion 27, and between zero degree region 27 and 180 degree region 29.These fine lines are coupled with the lines 12, 13, 14 in the binarymask of FIG. 1 for interconnection, while the blocking region 11prevents exposure of the fine lines during the exposure using the binarymask.

[0045]FIG. 3 shows the resulting fine lines 30, 31, 32 in the activeregion of the layout. The long narrow lines 12, 13, 14 interconnect thefine lines 30, 31, 32 with the flag shaped features 15, 16, 17. In theFig., the regions 35 and 36 do not print, but are higher intensityregions which show dark as artifacts of black and white printing of thecolor image generated using a simulation program.

[0046] Issues associated with this technique include the poor quality ofthe image of isolated lines, such as long line 12, and of the narrowspaces, such as between the flag shaped features 16 and 17. Classicaloptical proximity correction techniques can be applied to improvedimensional control of these images, however such processes according tothe prior art do not improve process latitude, making the structuresdifficult to manufacture.

[0047]FIGS. 4 and 5 show the binary mask and phase shift maskimplemented according to the present invention, extending phase shiftingtechniques to the more complex circuit pattern beyond the transistorgates in the active region. The binary mask of FIG. 4 is formed in aclear field 40. It includes blocking features 41 and 42. The patternelements which are common with FIG. 1 have like numbers, so theextensions 18, 19, 20 and the flag shaped features 15, 16, 17 have thesame reference numbers. A corresponding phase shift mask shown in FIG. 5includes an opaque field 50. The phase shifting regions have beenextended along the entire lengths of the lines excluding the extensions18, 19, 20 in this example. In addition, phase shifting in the region 49is used to assist the definition of the edges of the flag shaped regions16 and 17 in the narrow space between them. Thus, zero degree phaseshift regions 45 and 47 are formed, and 180 degree phase shift regions46 and 48 are formed. The phase shift regions 45, 46 and 47 extend tothe lower edges 51, 52 of the flag shaped regions 16, 17.

[0048] The portions of shifter 45 and shifter 47 extending in the area49 are sub-resolution, meaning that they are small enough in comparisonto the wavelength of the light used to expose the mask (λ), divided bythe numerical aperture NA of the beam (λ/NA) to not leave a trace in theresist. Their size depends on λ/NA but also on the resist used and theway it is processed. These features are out of phase compared to theshifter 46 also extending in the area 49. Their purpose is to improvethe aerial image of the printing feature 49. By adding some out-of-phaselight at the edge of the feature 47, the tail of the intensity profilecoming from feature 47 is partially removed, and the aerial imagebecomes sharper as can be seen on FIG. 6 compared to FIG. 3.

[0049] A simulation of an image resulting from application of the masksof FIGS. 4 and 5, is shown in FIG. 6, in which the regions 54, 55, 56and 57 are nonprinting artifacts as mentioned above of the black andwhite printing of the color simulation image. The long linescorresponding to the lines 12, 13, 14 of FIG. 1 are printed entirelyusing phase shifting, so that quality, narrow dimension features 51, 52and 53 result. The phase shifting assist feature between and on theedges of the flag shaped patterns 16, 17 results in better definition ofthe edges 58, 59 between the regions 16, 17. Thus, FIGS. 4-6 illustratethe application of phase shifting techniques to complex circuit patternbeyond the active regions of the device.

[0050]FIGS. 7, 8 and 9 illustrate a technique used for layout of complexstructures comprising an odd number of intersecting line segments usingphase shift masking. FIG. 7 shows a binary mask in a clear field 60comprising an opaque feature 61 corresponding to a first of intersectingline segments, an opaque feature 62 corresponding to a second of theintersecting line segments, and an opaque feature 63 corresponding to athird of the intersecting line segments. A corner cut-out region 64 isformed according to present technique is described further below. FIG. 8shows a phase shift mask in an opaque field 70 for formation of theintersecting line segments, and for use in combination with thecomplementary mask of FIG. 7. The phase shift mask includes 180 degreephase shift region 71, 180 degree phase shift region 72, zero degreephase shift region 73, and zero degree phase shift region 74. As can beseen, the 180 degree phase shift region 71 extends adjacent the linesegments corresponding to the regions 61 and 62 and around the cornerbetween regions 61 and 62. Also, the zero degree phase shift region 74extends adjacent to line segments and 62 and 63 and through the “corner”formed by the 180 degree angle in the intersection two line segments.The phase shift regions 72 and 73 extend along the line segment 63adjacent one side of the corner and along the other side 61 of thecorner, respectively and have opposite phases. An opaque feature is laidout in the corner between the two phase shift regions 72 and 73. Thecut-out feature 64 in the binary mask of FIG. 7 tends to expose theartifact which would be created by the phase transition in the cornerbetween phase shift regions 72 and 73.

[0051]FIG. 9 shows the simulation of the image printed using the phaseshift mask of FIG. 8, with a binary mask of FIG. 7. The features 81, 82,83 and 84 are nonprinting artifacts of the simulation program. The “T”shaped feature 85 results from the phase shift masking technique withcorner cutting. As can be seen, the narrow lines are formed withrelatively uniform thickness and straight sides. In the corner 86 whichcorresponds to the cut-out feature 64 of FIG. 7, the feature 85 isslightly less sharp than in the other corners. The shape of the printedcorner could be improved by applying some correction to the cut-out 64and the shifters 72 and 73.

[0052]FIGS. 10 and 11 illustrates the “corner cutting” technique asapplied to a structure comprising five intersecting line segments. Thus,FIG. 10 shows a binary mask 100 including an opaque feature havingblocking structure 101 corresponding to a first line segment, blockingstructure 102 corresponding to a second line segment, blocking structure103 corresponding to a third line segment, blocking structure 104corresponding to the fourth line segment, and blocking structure 105corresponding to the fifth line segment. A corner cutout feature 106 isformed between the line segments 101 and 105.

[0053]FIG. 11 shows the phase shift mask for use in combination with thebinary mask of FIG. 10. The phase shift mask of FIG. 11 is formed in anopaque field 110. 180 degree phase shift regions 111, 112 and 113 arelaid out in an alternating fashion as shown FIG. 11. Zero degree phaseshift regions 114, 115 and 116 are laid out in a complementary fashionto define the five intersecting line segments. An opaque feature isformed between the phase shift regions 114 and 113. The artifact whichwould be created by the phase transition between the phase shift regions113 and 114 is exposed by the cut-out 106 in the binary mask of FIG. 10.In addition, the shape of the opaque feature in the phase shift maskbetween the phase shift regions 113 the shape of the art-out 106 canalso be optimized and 114 can be modified using optical proximitycorrection techniques to improve that resulting image. The shape of thecut-out 106 can also be optimized.

[0054] A structure and a process for controlling phase mismatches oninside corners of complex structures is provided. Inside corner cut-outsare formed on the binary masks to block artifacts of phase transition inthe corner, and phase shift regions are adjusted by dividing them intofirst and second phase shift regions of opposite phase, and reshapingthem on inside corners to accommodate and optimize the effects of theinside corner extensions. The corners at which the extensions areapplied can be simply decided by applying them to all inside corners,when shapes of the corners are not critical. Alternatively, the cornerextensions can be applied only in one corner of a structure having anodd number of intersecting segments, such as one corner corresponding toa region in the layer characterized by greater process latitude thanother corners. The corner is picked, for example, by selecting an insidecorner having the greatest distance from an active area on the device,or an inside corner having a largest angle less than 180 degrees.

[0055] The selection of corners for the phase mismatch extensions mayaffect the assignment of zero and 180 degree phase shift regions. Thusit may be desirable to select the corners for inside corner extensionsprior to “coloring” the layout with phase assignments. A first approachto avoiding the corner conflicts is simply to select the phase shiftareas in a manner that does not cause a conflict. Of course this is notalways possible. Next, the conflicts can be left in regions on the chipwhere the design rules will tolerate the artifacts caused by the phasemismatch, or in other words, in regions characterized by greater processlatitude than the alternative locations, in the exposure patterns thatresult from the corner cut. In one example process, the cornerextensions are applied on all inside corners, then the layout is coloredto assigned phases, and then corners are rebuilt with optimized shapes.Alternatively, simplified phase assignment can be utilized when allcorners are provided with phase mismatch extensions.

[0056]FIGS. 12 and 13 illustrate problems encountered in the layout of aso-called double “T” structure. In FIG. 12, a phase shift mask in anopaque field 120 is shown for forming a double “T” structure havingvertical line segments 121 and 122 intersecting with horizontal linesegment 123. Vertical line segments 121 and 122 are close together, so asingle phase shift region 123 is formed between them. In this case,phase shift region 123 is a zero degree phase shift region. Phase shiftregion 124 beneath the line segment 123 is also a zero degrees phaseshift region creating a phase conflict in the region 129 between thevertical line segments 121 and 122. 180 degree phase shift regions 125,126, 127 and 128 are formed along the line segments in the corners asshown. The shapes of regions 125, 126, 127, 128 have not been optimizedin the corner in this example. The phase shift regions do not extend toall the way to the intersection of the line segments in this example.The phase mismatch in the region 129 can result in an aberration imagesuch that the quality of the line segments in that region is reduced.The assumption is that the distance between 121 and 122 is small enoughthat the printing of the region 129 will not be critical.

[0057]FIG. 13 illustrates a double “T” structure with vertical linesegments 131 and 132 formed in an opaque field 130. In this case,separate phase shift regions 133 and 134 are formed between the verticalline segments 131 and 132. A 180 degree phase shift region 135 is formedbetween them along the horizontal line segment 136. This resolves thephase mismatch which would have occurred with the zero degrees phaseshift region 137 according to the structure of FIG. 12, and allows forhigher quality printing of the images. In this case, the corner cuttingtechnique utilizes simple square shaped opaque features in the corners,rather than the diagonal shape shown in FIGS. 8 and 11. The square shapeof FIGS. 12 and 13 may be simpler to implement using a layout program ina processor with more limited power.

[0058]FIG. 14 provides a close-up of a portion of the layout of a phaseshift mask in an opaque field for a layer of an integrated circuitstructure. As can be seen, a comb shaped structure 141 is formed withzero degree phase shift regions (hatched, e.g. region 142) generally onthe upper and left and 180 degree regions (clear, e.g. region 143)generally on the lower and right. All inside corners are blocked withsquare opaque features (e.g. feature 144) in this example to minimizephase conflicts.

[0059] The generation of phase shift masks for a complex structure is anontrivial processing problem. Automatic assignment of phase shiftregions, and addition of optical proximity correction features andcorner features for preventing phase shift mismatches as described aboveare provided in this example to facilitate processing. Three stages inthe generation of phase shift mask layouts according to the processwhich is implemented using a design rule checking programming language(e.g. Vampire (TM) Design Rule Checker provided by Cadence DesignSystems, Inc.) as follows:

[0060] Definition of the Input Layers:

[0061] L13=layer(13 type(0))

[0062] L13 is the original poly layer

[0063] L12=layer(11 type(0))

[0064] L12 is the original poly layer shifted in the x and y directionby 0.02 micron

[0065] Generation of the Output Layers:

[0066] L2=geomSize(L13 −0.01 edges)

[0067] size L13 by −0.01 only edges (inner corners are not moved)

[0068] L2_(—)1=geomAndNot(L13 L2)

[0069] L2_(—)2=geomSize(L2_(—)1 0.01)

[0070] L3=geomAndNot(L2^(—)2 L13)

[0071] marker: 0.01 by 0.01 square in inner corners of L13

[0072] L4=geomSize(L13 0.01)

[0073] L5=geomSize(L13 0.01 edges)

[0074] size L13 by 0.01 only edges (outer corners are not moved)

[0075] L5_(—)1=geomAndNot(L4 L5)

[0076] L6=geomAndNot(L5_(—)1 L13)

[0077] marker: 0.01 by 0.01 square at the tips of outer corners

[0078] L6_(—)1=geomSize(L6 0.14)

[0079] L6_(—)2=geomSize(L13 0.15 edges)

[0080] L6^(—)3=geomAndNot(L6_(—)1 L6_(—)2)

[0081] L6_(—)4=geomSize(L6_(—)3 0.14)

[0082] L6_(—)5=geomSize(L6_(—)4 −0.14)

[0083] merges any 0.28 and below gaps

[0084] L6_(—)6=geomSize(L6_(—)5 −0.02)

[0085] L6_(—)7=geomSize(L6_(—)6 0.02) removes any 0.04 and belowgeometries

[0086] L7=geomAndNot(L6_(—)7 L13)

[0087] L7=layer to be removed from phase layer to cut the outer corners

[0088] L3_(—)1=geomSize(L3 0.15)

[0089] L8=geomAndNot(L3_(—)1 L13)

[0090] L8=layer to be removed from phase layer to cut the inner corners

[0091] L8_(—)1=geomOr(L7 L8)

[0092] add together the layers to be removed from the phase layer

[0093] L8_(—)2=geomSize(L13 −0.1)

[0094] L8_(—)3=geomSize(L8_(—)2 0.1)

[0095] removes any 0.2 micron and below geometries

[0096] L8_(—)4=geomAndNot(L13 L8_(—)3)

[0097] L13 without geometries larger than 0.2 micron

[0098] L9=geomSize(L8_(—)4 0.15)

[0099] L9_(—)1=geomAndNot(L9 L8_(—)1)

[0100] L9_(—)2=geomAndNot(L9_(—)1 L13)

[0101] L9_(—)3=geomSize(L9_(—)2 −0.03)

[0102] L10=geomSize(L9_(—)3 0.03)

[0103] −0.03/0.03 to remove any geometry below 0.06 micron

[0104] L10=phase shifter layer (no coloring performed)

[0105] L11=geomOverlap(L10 L12)

[0106] 0 degree phase-shift layer

[0107] L14=geomAndNot(L10 L11)

[0108] 180 degree phase-shift layer

[0109] A design rule checker can be utilized to identify all exposedfeatures (i.e. lines) and unexposed features (i.e. spaces between lines)of an input layout that have a size less than a minimum featuredimension. Features subject of the minimum feature dimension mayconstitute structures or spaces between structures. Different minimumfeature dimensions are applied to lines and to spaces in one embodiment.Thus, minimum feature structures can be identified by subtractingslightly more than ½ of a minimum feature dimension for lines from theoriginal size of an input structure. This results in eliminating allstructures which have a dimension less than the minimum dimension. Theremaining structures can then be reconstituted by adding slightly morethan ½ of the minimum dimension back. Minimum dimension structures canthen be identified by taking the original input structure andsubtracting all structures which result from the reconstitution step.This process can be characterized as performing a size down operation toeliminate small dimension features followed by a size up operation onremaining edges to produce a calculated layout. The small dimensionfeatures are then identified performing an “AND NOT” operation betweenthe original layout AND NOT and the calculated layout.

[0110] Narrow spaces can be identified by an opposite process. Inparticular, slightly more than ½ of the minimum feature dimension forspaces is added to the original size of the structure. This added lengthor width causes structures that are close together to overlap and merge.Next, the remaining structures are reconstituted by subtracting slightlymore than ½ of the minimum feature dimension from the sides ofstructures remaining. Narrow regions are identified by taking thereconstituted remaining structures and subtracting all originalstructures. Thus, a process can be characterized as performing a size upoperation to eliminate small dimension spaces, followed by a size downoperation on the remaining edges to produce a calculated layout. Thesmall dimension spaces are then identified by performing an “AND NOT”operation between the calculated layout and the original layout.

[0111] The next step in the procedure for automatic generation of phaseshift mask layouts involves identifying all corners in the structure.Inside corners and outside corners are identified. Outside corners areblocked to define ends of phase shift regions. Inside corners may resultin a phase mismatches discussed above. Inside corners are blocked, andthus provided with an extension of the opaque region, such as a squareextension, and a shortening of the phase shift regions so that they donot extend all the way to the inside corner. This square extension isapplied in all inside corners, whether a phase mismatch is found or not.Alternatively, the extension is applied only where phase mismatchesoccur.

[0112] Phase shift regions are formed in a simple case, by copying theinput structures in the minimum dimension features, and shifting up andto the left for 180 degree (or zero degree) shifters, and down and tothe right for zero degree (or 180 degree) shifters. The blocking regionsformed for the outside corners cut the shifted regions at the ends ofthe input structures, and the blocking structures formed on the insidecorners cut the shifted regions at the inside corners of the structureto provide well formed phase shift mask definitions. The phase“coloring” can be applied to the resulting phase shift regions in otherways, including manually, so that the zero and 180 degree regions areproperly laid out.

[0113] The limitation of this simple technique is that the shifts in theX and Y directions need to be carefully chosen if there is any polygonat an angle different from 0 to 90°.

[0114] All inside corners are blocked in the example shown in FIG. 14.However, in a preferred system, inside corners for which no phaseconflict is encountered would be filled with a phase shift region.

[0115] In another embodiment, the inside corner extensions which blockphase mismatches, are not applied on inside corners adjacent activeregions of devices that are near the corners, if a choice is possible.For structures having an odd number of segments intersecting, thelocation of the phase mismatch, and application of the corner extension,can be chosen at the angle farthest from the active regions in thedevice, or at the largest angle.

[0116] Once the inside corner extensions are identified, the extensionscan be optimally shaped to improve the resulting exposure pattern, suchas by changing the squares to diagonally shaped regions shown in FIGS. 8and 11. Other principles of optical proximity correction can be appliedto enhance the shapes of the inside corner extensions. Likewise, thephase shift regions can be shaped adjacent the inside corners to enhanceperformance. In one example system transitions may be enhanced betweenthe phase shift regions by placing a 90 degree phase shift regionbetween conflicting zero and 180 degree phase shift regions.

[0117]FIGS. 15A and 15B illustrate a prior art technique for laying outan array of dense shapes, such as a capacitor plate array in the layoutof a dynamic random access memory device. A phase shift mask as shown inFIG. 15A is used to form the array. The phase shift mask includes acolumn 200 of alternating phase transparent areas within an opaque field201. Likewise adjacent columns alternate in phase in a complementarymanner as shown. This results in the printing of lines on transitionsbetween the alternating phase shift areas and exposing regions insidethe phase shift regions. FIG. 15B illustrates the simulation of theexposure pattern. As can be seen, a dense array of oval patterns iscaused by the layout of FIG. 15A. For a denser array, it is desirable tomake the exposed patterns more rectangular in shape.

[0118]FIG. 16A illustrates an adjustment to the phase shift layoutaccording to the present invention to make the exposed patterns morerectangular. According to this technique, the phase shift regions havebeen adjusted so that they consist of a first phase shift area 215 and asecond phase shift area 216 having the same phase with an opaquesub-resolution feature 217 in between. Likewise, all of the phase shiftregions have been split into two phase shift regions as shown withsub-resolution features in between. Note that the assist feature whichdivides the phase shift region is not necessarily smaller than the phaseshift region. Lines are printed at the phase transitions, and thesub-resolution features between the like-phase regions do not print. Theresulting pattern is shown in FIG. 16B, where the exposure showsfeatures having much straighter sides and covering much greater areathan those of FIG. 15B. In the simulation plot of FIG. 16B, the darkoutlines, such as line 211, illustrate the final contour of the exposedregion. Thus, a technique for improving the images which result from useof phase shift areas involves adjusting a phase shift area having aparticular phase into a first phase shift area and a second phase shiftarea having the same particular phase and adding a sub-resolutionfeature in between.

[0119]FIGS. 17A and 17B illustrate the use of sub-resolution featureswithin the phase shift regions according to another technique of thepresent invention. In FIG. 17A, an opaque field 250 is shown with afirst phase shift region 251 and a second phase shift region 252 havingan opposite phase. Sub-resolution assist features 253 and 254 are formedwithin the phase shift region 251. Sub-resolution assist features 255and 256 are formed within the phase shift region 252. As can be seen,the phase shift regions 251 and 252 have respective perimeters. Thesub-resolution features 253, 254, 255, 256 are inside of the phase shiftregions and do not contact the perimeters in this example.

[0120]FIG. 17B shows simulation of the exposure patterns resulting fromthe phase shift mask of FIG. 17A. In the top, images 260 and 261 areshown which correspond to the use of the phase shift mask of FIG. 17A.Images 262 and 263 correspond to the use of the phase shift mask of FIG.17A without the sub-resolution assist features 253-256. As can be seen,with the sub-resolution assist features 253-256, the lines are muchstraighter and the exposure patterns are much more uniform. According toone technique, the sub-resolution features are placed within the phaseshift regions by first simulating the exposure patterns without thesub-resolution assist features. Hot spots, such as hot spot 264 in thesimulation image 263 or other anomalies, are identified. Sub-resolutionfeatures are then placed over the anomolies. Thus, sub-resolutionfeature 255 corresponds to the hot spot 264.

[0121] The techniques for improving phase shift masking for complexlayouts outlined above are combined into a process for producing phaseshift layout data and manufacturing phase shift masks for complexlayouts, as shown in FIG. 18. The process is also extended to themanufacturing of integrated circuits with improved structures. Thus,according to the present invention, the manufacturing process involvesreading a layout file which defines a complex layer of an integratedcircuit (step 300). For example, in one embodiment the layer comprisespolysilicon or another conductive material used as transition gates andinterconnect structures. Next, features to be left unexposed by the maskare identified which have a dimension less than a first particular value(step 301). Then, features to be exposed and having a dimension less thesecond particular value are identified (step 302). The first and secondparticular values may be the same value or different, as suits theparticular implementation.

[0122] Next, the process involves laying out phase shift regions for theidentified features according to a design rule (step 303). One exampledesign rule involves laying out phase shift regions having a zero degreephase (or 180 degree phase) to the upper left, and a phase shift regionshaving the opposite phase, such as 180 degree phase (or zero degreephase) to lower right. This simple phase shift layout rule results inphase conflicts, where adjacent phase shift regions have the same phaseso phase transitions do not occur. Any other phase assignment techniquecan be used. The phase conflicts are identified in a next step (step304). Adjustments are applied to the phase shift regions based onidentified phase conflicts (step 305). For example, the corner cuttingtechnique described with respect to FIGS. 7-11 is applied. In a nextstep, the exposure pattern is simulated and assist features are added tothe phase shift regions based on the simulation (step 306). Rather thanusing simulation for placement of sub-resolution assist features, thelocations of the sub-resolution features can be determined based ondesign rules. For example, one design rule is to place place a 0.1 μmsquare assist feature, 0.2 μm away from the edge of the phase shiftregion. Thus, phase shift regions may be adjusted using sub-resolutionassist features within the perimeter of the phase shift region, or bydividing the phase shift region as described with reference to FIGS. 16Aand 17A.

[0123] In a next step, other optical proximity correction techniques areapplied and the phase shift mask layout is completed (step 307). Acomplementary mask is then laid out, including the corner cut-outs asnecessary for intersecting line segments and the like (step 308).

[0124] With the completed phase shift and complementary mask layouts,the masks are printed using techniques known in the art (step 309). See,U.S. Pat. Nos. 6,096,458; 6,057,063; 5,246,800; 5,472,814; and5,702,847, which provide background material for phase shift maskmanufacturing. Finally, integrated circuits are manufactured using theresulting phase shift masks (step 310).

[0125] Overall, the embodiments described provide a solution forapplying phase shift masks extensively in integrated circuit layouts.This provides for shrinking entire layouts or significant portions oflayouts. The process involves first identifying features using acomputer program to define any features that have a dimension which issmaller than a specified minimum dimension. Also, the process is appliedto identify spaces between features which are smaller than a minimumdimension. The minimum dimension for spacing may be different than theminimum dimension for structures. After detection of features smallerthan a minimum dimension, phase shift regions are assigned. Non-printingphase shift regions can be used for providing greater contrast in narrowisolated spaces. Inside corner extensions to block phase conflicts areadded where necessary. Complementary trim masks are generated usingestablished techniques. Finally, optical proximity correction modelingis used to optimize the shapes being implemented.

[0126] Embodiments of the invention also provides techniques forapplying phase shifting to specific shapes, such as “T” shapes, “Y”shapes, “U” shapes and “double T” shapes.

[0127] Optical proximity correction can be applied to the resultingphase shifted layouts. Serifs can be added to corners, line sizes can beadjusted, hammer heads can be added, phase shift areas can be sized, andassist opaque bars may be added to phase shift areas, using opticalproximity correction modeling techniques.

[0128] The foregoing description of various embodiments of the inventionhave been presented for purposes of illustration and description. Thedescription is not intended to limit the invention to the precise formsdisclosed. Many modifications and equivalent arrangements will beapparent to people skilled in the art.

What is claimed is:
 1. A method, comprising: identifying a pattern for alayer to be formed using a photolithographic mask, the pattern includingfeatures having a dimension smaller than a first particular featuresize, and at least one relatively large feature, the at least onerelatively large feature and another feature in the pattern havingrespective sides separated by a space between said respective sides,said space having a dimension less than a second particular featuresize; and laying out phase shift regions for the identified pattern toproduce a phase shift mask, the phase shift mask having a first set ofphase shift regions to define said features having a dimension smallerthan the first particular feature size, and a second set of phase shiftregions to assist definition of said respective side of said at leastone relatively large feature.
 2. The method of claim 1, including:laying out a complementary mask including one or more opaque featurespreventing exposure of said features having a dimension smaller than thefirst particular feature size, and at least one opaque feature used todefine said at least one relatively large feature.
 3. The method ofclaim 1, wherein said at least one relatively large feature has aperimeter including one of said respective sides, and the second set ofphase shift regions includes a first phase shift region inside theperimeter of said at least one relatively large feature, and whereinsaid first phase shift region comprises a sub-resolution region.
 4. Themethod of claim 1, wherein said at least one relatively large featureand said other feature have perimeters including said respective sides,and the second set of phase shift regions includes a first phase shiftregion inside the perimeter of said at least one relatively largefeature, and a second phase shift region inside the perimeter of saidother feature, and wherein said first and second phase shift regionscomprise sub-resolution regions.
 5. The method of claim 1, wherein saidat least one relatively large feature and said other feature haveperimeters including said respective sides, and the second set of phaseshift regions includes a first phase shift region inside the perimeterof said at least one relatively large feature, a second phase shiftregion inside the perimeter of said other feature, and a third phaseshift region having a different phase than said first phase between saidrespective sides, and wherein said first and second shift regionscomprise sub-resolution regions.
 6. The method of claim 5, wherein saiddifferent phase is about 180 degrees out of phase relative to said firstphase.
 7. The method of claim 1, wherein said at least one relativelylarge feature intersects with one of said features having a dimensionless than a first particular feature size, and wherein a phase shiftregion having a first phase in said first set of phase shift regionsintersects with a phase shift region in said space in said second set ofphase shift regions.
 8. The method of claim 1, wherein said at least onerelatively large feature has a perimeter including one of saidrespective sides, and wherein said perimeter of said at least onerelatively large feature intersects with one of said features having adimension less than a first particular feature size, and wherein a phaseshift region having a first phase in said first set of phase shiftregions intersects with a phase shift region in said space in saidsecond set of phase shift regions, and a phase shift region having adifferent phase in said first set of phase shift regions intersects witha sub-resolution phase shift region in said second set of phase shiftregions inside said perimeter of said at least one relatively largefeature.
 9. The method of claim 1, wherein the pattern includes one ormore features having a feature size equal to a critical dimension, andwherein said critical dimension is smaller than at least one of saidfirst particular feature size and said second particular feature size.10. The method of claim 1, wherein said identifying features includesreading a layout file which identifies features of the pattern, andprocessing the layout file.
 11. The method of claim 1, wherein saidphase shift mask includes an opaque field, and said phase shift regionsin said first and second sets of phase shift regions include a pluralityof transparent regions having a first phase within said opaque field,and a plurality of complementary transparent regions having a differentphase approximately 180 degrees out of phase with respect to the firstphase, within said opaque field.
 12. The method of claim 2, wherein saidcomplementary mask comprises a binary mask.
 13. The method of claim 1,including producing a machine readable layout file defining the layoutof the phase shift mask.
 14. The method of claim 13, including producingthe phase shift mask.
 15. The method of claim 14, including producing anintegrated circuit using the phase shift mask.
 16. A data processingsystem, comprising: a processor; a machine readable data storage mediumcoupled to the processor, having stored thereon instructions executableby the processor defining steps for laying out a photolithographic maskthat defines a pattern for a layer to be formed using aphotolithographic mask, the pattern including features having adimension smaller than a first particular feature size, and at least onerelatively large feature, the at least one relatively large feature andanother feature in the pattern having respective sides separated by aspace between said respective sides, said space having a dimension lessthan a second particular feature size; the steps comprising: laying outphase shift regions for the identified pattern to produce a phase shiftmask, the phase shift mask having a first set of phase shift regions todefine said features having a dimension smaller than the firstparticular feature size, and a second set of phase shift regions toassist definition of said respective side of said at least onerelatively large feature.
 17. The data processing system of claim 16,the steps including: laying out a complementary mask including one ormore opaque features preventing exposure of said features having adimension smaller than the first particular feature size, and at leastone opaque feature used to define said at least one relatively largefeature.
 18. An article of manufacture, comprising: a machine readabledata storage medium, having stored thereon instructions executable by adata processing system defining steps for laying out a photolithographicmask that defines a pattern in a layer to be formed using the mask, thepattern including features having a dimension smaller than a firstparticular feature size, and at least one relatively large feature, theat least one relatively large feature and another feature in the patternhaving respective sides separated by a space between said respectivesides, said space having a dimension less than a second particularfeature size; the steps comprising: laying out phase shift regions forthe identified pattern to produce a phase shift mask, the phase shiftmask having a first set of phase shift regions to define said featureshaving a dimension smaller than the first particular feature size, and asecond set of phase shift regions to assist definition of saidrespective side of said at least one relatively large feature.
 19. Thearticle of claim 18, the steps including: laying out a complementarymask including one or more opaque features preventing exposure of saidfeatures having a dimension smaller than the first particular featuresize, and at least one opaque feature used to define said at least onerelatively large feature.
 20. A method for manufacturing aphotolithographic mask that defines a pattern in a layer to be formedusing the mask, wherein the pattern including features having adimension smaller than a first particular feature size, and at least onerelatively large feature, the at least one relatively large feature andanother feature in the pattern having respective sides separated by aspace between said respective sides, said space having a dimension lessthan a second particular feature size; the method comprising: laying outphase shift regions for the identified pattern to produce a phase shiftmask, the phase shift mask having a first set of phase shift regions todefine said features having a dimension smaller than the firstparticular feature size, and a second set of phase shift regions toassist definition of said respective side of said at least onerelatively large feature; and forming said phase shift pattern on aportion of mask substrate material.
 21. The article of claim 20, thesteps including: laying out a complementary mask including one or moreopaque features preventing exposure of said features having a dimensionsmaller than the first particular feature size, and at least one opaquefeature used to define said at least one relatively large feature.